This invention relates to an improved process for the production of ultrahigh carbon steels which have ultrafine grains and highly spheroidized carbides.
Production of such steels in general is described in a number of sources, including U.S. Pat. Nos. 3,951,697; 4,448,613; 4,533,390 and 4,769,214 to Sherby et al. Ultrahigh carbon steel (UHCS) alloys generally contain hypereutectoid carbon contents, and may contain small amounts of other elements including but not limited to chromium, silicon and/or aluminum. Typically, UHCS alloys, that comprise only small amounts of elements other than carbon and iron, have carbon contents very roughly intermediate between conventional hypoeutectoid carbon steels (about 0.1-0.8 wt. %) and cast irons (generally greater than 2.1 wt. %).
It has been found particularly desirable to produce UHCS alloys with superplastic properties; that is, having the ability to be deformed at elevated temperatures to exceptionally large tensile strains without fracturing. Such alloys may be used to manufacture objects of complicated shape by relatively simple forming procedures, rather than by assembly of many small parts involving machining and joining. The characteristic feature of superplastic materials is the microstructure. An ultrafine grain size (generally from about 1 to about 5 micrometers) is essential for superplasticity.
To obtain the ultrafine grains required for superplasticity of the UHCS alloys, there must be two phases present, one or each of which prevents the other from coarsening. In the UHCS alloys, the two phases are iron (ferrite) and iron carbide (cementite) or another carbon-rich phase. The processing of UHCS alloys to obtain steel with superplastic properties generally involves two stages. In the first stage, a relatively homogeneous material containing pearlite, a lamellar mixture of ferrite and cementite plates, is produced. In the second stage, the pearlite microstructure is converted or transformed to a superplastic microstructure wherein the carbides are predominantly spheroidized, and the ferrite is ultrafine-grained.
One technique used for producing the spheroidal type of microstructure required for superplasticity is termed "Divorced Eutectoid Transformation"("DET") and is described in U.S. Pat. No. 4,448,61.3. In that process, the steel is heated to a soaking temperature approximately 50.degree. C. above the A.sub.1 transformation temperature (i.e., slightly above the pearlite stability range), and held at that temperature for a time sufficient to dissolve the major portion of the carbides into the austenite matrix. In that step, according to the patent's disclosure, the carbon is distributed nonuniformly within the austenite. Then, the steel is cooled at a rate equivalent to air cooling. According to the '613 patent, for superplasticity it is desirable that the carbide particles in the alloy, after processing, remain finely distributed. The nondesirability of a continuous network of carbide particles is noted.
Deformation of the alloy may be carried out during the DET process. That variation is known as the DETWAD (Divorced Eutectoid Transformation With Associated Deformation) process.
Nakano et al., Transactions ISIJ, 17: 110-115 (1977) showed that the divorced eutectoid transformation (DET) could occur in steels having significantly less carbon than UHCS, upon slow cooling (20.degree. C./hour) from above the A.sub.1 transformation temperature.
The DET process functions well to produce superplastic steels when used with alloys having compositions as described in U.S. Pat. No. 4,448,613. However, more recently, other elements such as aluminum have been added to UHCS alloys, to increase their superplastic forming temperature while maintaining good hot ductility and cold workability, for instance as described in U.S. Pat. No. 4,769,214. In addition to aluminum, such alloys contain a stabilizing element which acts to stabilize iron carbides against graphitization. Such stabilizing elements can include up to about 2 weight % chromium or 0.4 weight % molybdenum. U.S. Pat. No. 4,769,214 states that the DET and DETWAD processes may be used satisfactorily to treat such alloys However in practice, a dramatically reduced fraction of spheroidal carbides was formed with such alloys. On investigation it was determined that the cooling rate equivalent to air cooling is a primary cause of the decrease in the fraction of spheroidal carbides and consequent decrease in desirable superplasticity qualities.